LoongArch: Fix incorrect code generation for sad pattern

LoongArch: Fix incorrect code generation for sad pattern

When I attempt to enable vect_usad_char effective target for LoongArch, slp-reduc-sad.c
and vect-reduc-sad*.c tests fail. These tests fail because the sad pattern generates bad
code. This patch to fixed them, for sad patterns, use zero expansion instead of sign
expansion for reduction.

Currently, we are fixing failed vectorized tests, and in the future, we will
enable more tests of "vect" for LoongArch.

gcc/ChangeLog:

	* config/loongarch/lasx.md: Use zero expansion instruction.
	* config/loongarch/lsx.md: Ditto.